/*
 * Copyright 2010-2012 Susanta Tewari. <freecode4susant@users.sourceforge.net>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

package bd.org.apache.commons.math.stat.descriptive.moment;

import bd.org.apache.commons.math.exception.NullArgumentException;
import bd.org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
import bd.org.apache.commons.math.util.MathUtils;

import java.io.Serializable;

/**
 * Computes the first moment (arithmetic mean).  Uses the definitional formula:
 * <p>
 * mean = sum(x_i) / n </p>
 * <p>
 * where <code>n</code> is the number of observations. </p>
 * <p>
 * To limit numeric errors, the value of the statistic is computed using the
 * following recursive updating algorithm: </p>
 * <p>
 * <ol>
 * <li>Initialize <code>m = </code> the first value</li>
 * <li>For each additional value, update using <br>
 *   <code>m = m + (new value - m) / (number of observations)</code></li>
 * </ol></p>
 * <p>
 *  Returns <code>Double.NaN</code> if the dataset is empty.</p>
 * <p>
 * <strong>Note that this implementation is not synchronized.</strong> If
 * multiple threads access an instance of this class concurrently, and at least
 * one of the threads invokes the <code>increment()</code> or
 * <code>clear()</code> method, it must be synchronized externally.</p>
 *
 * @version $Id: FirstMoment.java 1244107 2012-02-14 16:17:55Z erans $
 */
class FirstMoment extends AbstractStorelessUnivariateStatistic implements Serializable {

    /** Serializable version identifier */
    private static final long serialVersionUID = 6112755307178490473L;

    /** Count of values that have been added */
    protected long n;

    /** First moment of values that have been added */
    protected double m1;

    /**
     * Deviation of most recently added value from previous first moment.
     * Retained to prevent repeated computation in higher order moments.
     */
    protected double dev;

    /**
     * Deviation of most recently added value from previous first moment,
     * normalized by previous sample size.  Retained to prevent repeated
     * computation in higher order moments
     */
    protected double nDev;

    /**
     * Create a FirstMoment instance
     */
    public FirstMoment() {

        n    = 0;
        m1   = Double.NaN;
        dev  = Double.NaN;
        nDev = Double.NaN;
    }

    /**
     * Copy constructor, creates a new {@code FirstMoment} identical
     * to the {@code original}
     *
     * @param original the {@code FirstMoment} instance to copy
     */
    public FirstMoment(FirstMoment original) {

        super();

        copy(original, this);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void increment(final double d) {

        if (n == 0) {
            m1 = 0.0;
        }

        n++;

        double n0 = n;

        dev  = d - m1;
        nDev = dev / n0;
        m1   += nDev;
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public void clear() {

        m1   = Double.NaN;
        n    = 0;
        dev  = Double.NaN;
        nDev = Double.NaN;
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public double getResult() {

        return m1;
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public long getN() {

        return n;
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public FirstMoment copy() {

        FirstMoment result = new FirstMoment();

        copy(this, result);

        return result;
    }


    /**
     * Copies source to dest.
     * <p>Neither source nor dest can be null.</p>
     *
     * @param source FirstMoment to copy
     * @param dest FirstMoment to copy to
     * @throws NullArgumentException if either source or dest is null
     */
    public static void copy(FirstMoment source, FirstMoment dest) throws NullArgumentException {

        MathUtils.checkNotNull(source);
        MathUtils.checkNotNull(dest);
        dest.setData(source.getDataRef());

        dest.n    = source.n;
        dest.m1   = source.m1;
        dest.dev  = source.dev;
        dest.nDev = source.nDev;
    }
}
